Retroreflective cube corner sheeting mold and sheeting formed therefrom

ABSTRACT

Laminae suitable for use in a mold suitable for use in forming retroreflective sheeting and methods of making such laminae are disclosed. A representative lamina includes a single row of optically opposing cube corner elements disposed on its working surface. The working surface of a lamina is provided with a plurality of cube corner elements formed by the optical surfaces defined by three groove sets. Corresponding surfaces of opposing groove sets intersect substantially orthogonally along a reference edge to define first and second optical surfaces of the respective cube corner elements. The third optical surface of each respective cube corner element is defined by one surface of the third groove set. 
     The laminae can inclued at least one cube corner element having a four-sided perimeter in plan view in which a first and second pair of opposed paralled sides are obliquely disposed with respect to each other.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of pending U.S. application Ser. No. 09/342,410,filed Jun. 29, 1999 a now U.S. Pat. No. 6,120,881, which is acontinuation of U.S. application No. Ser. 08/886,998, filed Jul. 2, 1997and now issued as U.S. Pat. No. 5,981,032.

FIELD OF THE INVENTION

The present invention relates generally to molds suitable for use informing cube corner retroreflective sheeting, to methods for making thesame, and to retroreflective sheeting formed from such molds. Inparticular, the invention relates to molds formed from a plurality ofthin laminae and to methods for making the same.

BACKGROUND OF THE INVENTION

Retroreflective materials are characterized by the ability to redirectlight incident on the material back toward the originating light source.This property has led to the wide-spread use of retroreflective sheetingin a variety of conspicuity applications. Retroreflective sheeting isfrequently used on flat, rigid articles such as, for example, road signsand barricades, however it is also used on irregular or flexiblesurfaces. For example, retroreflective sheeting can be adhered to theside of a truck trailer, which requires the sheeting to pass overcorrugations and protruding rivets, or the sheeting can be adhered to aflexible body portion such as a road worker's safety vest or other suchsafety garment. In situations where the underlying surface is irregularor flexible, the retroreflective sheeting desirably possesses theability to conform to the underlying surface without sacrificingretroreflective performance. Additionally, retroreflective sheeting isfrequently packaged and shipped in roll form, thus requiring thesheeting to be sufficiently flexible to be rolled around a core.

Two known types of retroreflective sheeting are microsphere-basedsheeting and cube corner sheeting. Microsphere-based sheeting, sometimesreferred to as “beaded” sheeting, employs a multitude of microspherestypically at least partially embedded in a binder layer and havingassociated specular or diffuse reflecting materials (e.g., pigmentparticles, metal flakes or vapor coats, etc.) to retroreflect incidentlight. Illustrative examples are disclosed in U.S. Pat. No. 3,190,178(McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No.5,066,098 (Kult). Advantageously, microsphere-based sheeting cangenerally be adhered to corrugated or flexible surfaces. Also, due tothe symmetrical geometry of beaded retroreflectors, microsphere basedsheeting exhibits a relatively orientationally uniform total lightreturn when rotated about an axis normal to the surface of the sheeting.Thus, such microsphere-based sheeting has a relatively low sensitivityto the orientation at which the sheeting is placed on a surface. Ingeneral, however, such sheeting has a lower retroreflective efficiencythan cube corner sheeting.

Cube corner retroreflective sheeting comprises a body portion typicallyhaving a substantially planar base surface and a structured surfacecomprising a plurality of cube corner elements opposite the basesurface. Each cube-corner element comprises three mutually substantiallyperpendicular optical faces that intersect at a single reference point,or apex. The base of the cube corner element acts as an aperture throughwhich light is transmitted into the cube corner element. In use, lightincident on the base surface of the sheeting is refracted at the basesurface of the sheeting, transmitted through the bases of the cubecorner elements disposed on the sheeting, reflected from each of the ofthe three perpendicular cube-corner optical faces, and redirected towardthe light source. The symmetry axis, also called the optical axis, of acube corner element is the axis that extends through the cube cornerapex and forms an equal angle with the three optical faces of the cubecorner element. Cube corner elements typically exhibit the highestoptical efficiency in response to light incident on the base of theelement roughly along the optical axis. The amount of lightretroreflected by a cube corner retroreflector drops as the incidenceangle deviates from the optical axis.

The maximum retroreflective efficiency of cube corner retroreflectivesheeting is a function of the geometry of the cube corner elements onthe structured surface of the sheeting. The terms ‘active area’ and‘effective aperture’ are used in the cube corner arts to characterizethe portion of a cube corner element that retroreflects light incidenton the base of the element. A detailed teaching regarding thedetermination of the active aperture for a cube corner element design isbeyond the scope of the present disclosure. One procedure fordetermining the effective aperture of a cube corner geometry ispresented in Eckhardt, Applied Optics, v. 10, n. 7, July, 1971, pp.1559-1566. U.S. Pat. No. 835,648 to Straubel also discusses the conceptof effective aperture. At a given incidence angle, the active area canbe determined by the topological intersection of the projection of thethree cube corner faces onto a plane normal to the refracted incidentlight with the projection of the image surfaces for the thirdreflections onto the same plane. The term ‘percent active area’ is thendefined as the active area divided by the total area of the projectionof the cube corner faces. The retroreflective efficiency ofretroreflective sheeting correlates directly to the percentage activearea of the cube corner elements on the sheeting.

Additionally, the optical characteristics of the retroreflection patternof retroreflective sheeting are, in part, a function of the geometry ofthe cube corner elements. Thus, distortions in the geometry of the cubecorner elements can cause corresponding distortions in the opticalcharacteristics of the sheeting. To inhibit undesirable physicaldeformation, cube corner elements of retroreflective sheeting aretypically made from a material having a relatively high elastic modulussufficient to inhibit the physical distortion of the cube cornerelements during flexing or elastomeric stretching of the sheeting. Asdiscussed above, it is frequently desirable that retroreflectivesheeting be sufficiently flexible to allow the sheeting to be adhered toa substrate that is corrugated or that is itself flexible, or to allowthe retroreflective sheeting to be wound into a roll to facilitatestorage and shipping.

Cube corner retroreflective sheeting is manufactured by firstmanufacturing a master mold that includes an image, either negative orpositive, of a desired cube corner element geometry. The mold can bereplicated using nickel electroplating, chemical vapor deposition orphysical vapor deposition to produce tooling for forming cube cornerretroreflective sheeting. U.S. Pat. No. 5,156,863 to Pricone, et al.provides an illustrative overview of a process for forming tooling usedin the manufacture of cube corner retroreflective sheeting. Knownmethods for manufacturing the master mold include pin-bundlingtechniques, direct machining techniques, and laminate techniques. Eachof these techniques has benefits and limitations.

In pin bundling techniques, a plurality of pins, each having a geometricshape on one end, are assembled together to form a cube-cornerretroreflective surface. U.S. Pat. No. 1,591,572 (Stimson), U.S. Pat.No. 3,926,402 (Heenan), U.S. Pat. No. 3,541,606 (Heenan et al.) and U.S.Pat. No. 3,632,69 (Howell) provide illustrative examples. Pin bundlingtechniques offer the ability to manufacture a wide variety of cubecorner geometries in a single mold. However, pin bundling techniques areeconomically and technically impractical for making small cube cornerelements (e.g. less than about 1.0 millimeters).

In direct machining techniques, a series of grooves is formed in aunitary substrate to form a cube-corner retroreflective surface. U.S.Pat. No. 3,712,706 (Stamm) and U.S. Pat. No. 4,588,258 (Hoopman) provideillustrative examples. Direct machining techniques offer the ability toaccurately machine very small cube corner elements which are compatiblewith flexible retroreflective sheeting. However, it is not presentlypossible to produce certain cube corner geometries that have very higheffective apertures at low entrance angles using direct machiningtechniques. By way of example, the maximum theoretical total lightreturn of the cube corner element geometry depicted in U.S. Pat. No.3,712,706 is approximately 67%.

In laminate techniques, a plurality of laminae, each lamina havinggeometric shapes on one end, are assembled to form a cube-cornerretroreflective surface. German Provisional Publication (OS) 19 17 292,International Publication Nos. WO 94/18581 (Bohn, et al.), WO 97/04939(Mimura et al.), and WO 97/04940 (Mimura et al.), all disclose a moldedreflector wherein a grooved surface is formed on a plurality of plates.The plates are then tilted by a certain angle and each second plate isshifted crosswise. This process results in a plurality of cube cornerelements, each element formed by two machined surfaces and one sidesurface of a plate. German Patent DE 42 36 799 to Gubela discloses amethod for producing a molding tool with a cubical surface for theproduction of cube corners.

An oblique surface is ground or cut in a first direction over the entirelength of one edge of a band. A plurality of notches are then formed ina second direction to form cube corner reflectors on the band. Finally,a plurality of notches are formed vertically in the sides of the band.German Provisional Patent 44 10 994 C2 to Gubela is a related patent.

BRIEF SUMMARY OF THE INVENTION

The present application discloses a master mold suitable for use informing retroreflective sheeting from a plurality of laminae and methodsof making the same. Advantageously, master molds manufactured accordingto the present teachings enable the manufacture of retroreflective cubecorner sheeting that exhibits retroreflective efficiency levelsapproaching 100%. To facilitate the manufacture of flexibleretroreflective sheeting, the disclosed methods enable the manufactureof cube corner retroreflective elements having a width as small as 0.010millimeters. Additionally, the disclosure teaches the manufacture of acube corner retroreflective sheeting that exhibits symmetricalretroreflective performance in at least two different orientations.Efficient, cost-effective methods of making molds formed from aplurality of laminae are also disclosed.

One embodiment relates to a lamina suitable for use in a mold for use informing retroreflective cube corner articles, the lamina having opposingfirst and second major surfaces defining therebetween a first referenceplane, the lamina further including a working surface connecting thefirst and second major surfaces, the working surface defining a secondreference plane substantially parallel to the working surface andperpendicular to the first reference plane and a third reference planeperpendicular to the first reference plane and the second referenceplane. The lamina includes: (a) a first groove set including at leastone V-shaped groove in the working surface of the lamina, the groovedefining a first groove surface and a second groove surface thatintersect to define a first groove vertex; (b) a second groove setincluding at least one V-shaped groove in the working surface of thelamina, the groove defining a third groove surface and a fourth groovesurface that intersect to define a second groove vertex, the thirdgroove surface intersecting the first groove surface substantiallyorthogonally to define a first reference edge; and (c) a third grooveset including at least two parallel adjacent V-shaped grooves in theworking surface of the lamina, each groove defining a fifth groovesurface and a sixth groove surface that intersect to define a thirdgroove vertex, the fifth groove surface intersecting substantiallyorthogonally with the first and third groove surfaces to form at leastone cube corner disposed in a first orientation.

In one embodiment, the first and second groove sets are formed such thatthe respective first and third groove surfaces intersect approximatelyorthogonally to define reference edges, and the second and fourth groovesurfaces intersect approximately orthogonally to define reference edges,that are substantially parallel to the first reference plane. Finally,the third groove set comprises a plurality of grooves having respectivevertices that extend along an axis perpendicular to the first referenceplane. In this embodiment, the lamina comprises a single row ofoptically opposing cube corner elements disposed on the working surfaceof the lamina.

The three mutually perpendicular optical faces of each cube cornerelement are preferably formed on a single lamina. All three opticalfaces are preferably formed by the machining process to ensure opticalquality surfaces. A planar interface is preferably maintained betweenadjacent laminae during the machining phase and subsequent thereto so asto minimize alignment problems and damage due to handling of thelaminae.

A method is disclosed for manufacturing a lamina for use in a moldsuitable for use in forming retroreflective cube corner articles, thelamina having opposing first and second major surfaces definingtherebetween a first reference plane, the lamina further including aworking surface connecting the first and second major surfaces, theworking surface defining a second reference plan substantially parallelto the working surface and perpendicular to the first reference planeand a third reference lane perpendicular to the first reference planeand the second reference plane. The method includes: (a) forming a firstgroove set including at least one V-shaped groove in the working surfaceof the lamina, the groove defining a first groove surface and a secondgroove surface that intersect to define a first groove vertex; (b)forming a second groove set including at least one V-shaped groove inthe working surface of the lamina, the groove defining a third groovesurface and a fourth groove surface that intersect to define a secondgroove vertex, the third groove surface intersecting the first groovesurface substantially orthogonally to define a first reference edge; and(c) forming a third groove set including at least two parallel adjacentV-shaped grooves in the working surface of the lamina, each groovedefining a fifth groove surface and a sixth groove surface thatintersect to define a third groove vertex, a fifth groove surfaceintersecting substantially orthogonally with the first and third groovesurfaces to form at least one cube corner disposed in a firstorientation.

A preferred mold assembly includes a plurality of laminae, the laminaeincluding opposed parallel first and second major surfaces definingtherebetween a first reference plane, each lamina further including aworking surface connecting the first and second major surfaces, theworking surface defining a second reference plane substantially parallelto the working surface and perpendicular to the first reference planeand a third reference plane perpendicular to the first reference planeand the second reference plane. The working surface of a plurality ofthe laminae includes: (a) a first groove set including at least twoparallel adjacent V-shaped grooves in the working surface of each of thelaminae, a plurality of the adjacent grooves defining a first groovesurface and a second groove surface that intersect to define a firstgroove vertex; (b) a second groove set including at least two paralleladjacent V-shaped grooves in the working surface of each of the laminae,a plurality of the adjacent grooves defining a third groove surface anda fourth groove surface that intersect to define a second groove vertex,the third groove surface intersecting the first groove surfacesubstantially orthogonally to define a first reference edge; and (c) athird groove set including at least two parallel adjacent V-shapedgrooves in the working surface of the laminae, the third groove defininga fifth groove surface and a sixth groove surface that intersect todefine a third groove vertex, the fifth groove surface intersectingsubstantially orthogonally with the first and third groove surfaces toform at least one cube corner disposed in a first orientation.

In one embodiment, the first and second groove sets are formed such thattheir respective vertices extend along axes that, in a top plan view,are perpendicular to the respective first reference planes. Finally, thethird groove set comprises a plurality of grooves having respectivevertices that extend along axes perpendicular to the first referenceplane. In this embodiment, each lamina comprises a single row ofoptically opposing cube corner elements disposed on the working surfaceof the lamina.

Also disclosed is a method of manufacturing a plurality of laminae foruse in a mold suitable for use in forming retroreflective cube cornerarticles, each lamina having opposing first and second major surfacesdefining therebetween a first reference plane, each lamina furtherincluding a working surface connecting the first and second majorsurfaces, the working surface defining a second reference planesubstantially parallel to the working surface and perpendicular to thefirst reference plane and a third reference plane perpendicular to thefirst reference plane and the second reference plane. The methodincludes: (a) orienting a plurality of laminae to have their respectivefirst reference planes parallel to each other and disposed at a firstangle relative to a fixed reference axis; (b) forming a first groove setincluding a plurality of V-shaped grooves in the working surface of thelamina, the respective grooves defining a first groove surface and asecond groove surface that intersect to define a first groove vertex;(c) orienting the plurality of laminae to have their respective firstreference planes parallel to each other and disposed at a second anglerelative to the fixed reference axis; (d) forming a second groove setincluding a plurality of V-shaped grooves in the working surface of thelamina, the respective grooves defining a third groove surface and afourth groove surface that intersect to define a second groove vertex,the respective third groove surfaces intersecting the first groovesurfaces substantially orthogonally to define a first reference edge;and (e) forming a third groove set including a plurality of V-shapedgrooves in the working surface of the lamina, the respective thirdgrooves defining a fifth groove surface and a sixth groove surface thatintersect to define a third groove vertex, the fifth groove surfaceintersecting substantially orthogonally with the first and third groovesurfaces to form at least one cube corner disposed in a firstorientation.

In one disclosed method, the plurality of laminae are assembled in asuitable fixture that defines a base plane. Preferably, the fixturesecures the laminae such that their respective first reference planesare substantially parallel and are disposed at a first angle thatpreferably measures between 45° and 90°, and more preferably measuresbetween 45° and 60° relative to a fixed reference axis that is a normalvector to the base plane. The first groove set is then formed byremoving portions of each of the plurality of lamina proximate theworking surface of the plurality of laminae by using a suitable materialremoval technique such as, for example, ruling, fly-cutting, grinding,or milling. The plurality of laminae are then reassembled in the fixtureand secured such that their respective first reference planes aresubstantially parallel and are disposed at a second angle of between 45°and 90°, and more preferably between 45° and 60° relative to a fixedreference axis that is a normal vector to the base plane. The secondgroove set is then formed using suitable material removal techniques asdescribe above. The plurality of laminae are then reassembled in thefixture and secured such that their respective first reference planesare substantially parallel to the reference axis. The third groove setis then formed using suitable material removal techniques as describedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a single lamina suitable for use in thedisclosed methods.

FIG. 2 is an end view of a single lamina following a first machiningstep.

FIG. 3 is a side view of a single lamina following a first machiningstep.

FIG. 4 is a top view of a single lamina following a first machiningstep.

FIG. 5 is an end view of a single lamina following a second machiningstep.

FIG. 6 is a side view of a single lamina following a second machiningstep.

FIG. 7 is a top view of a single lamina following a second machiningstep.

FIG. 8 is a perspective view of a single lamina following a secondmachining step.

FIG. 9 is an end view of a single lamina following a third machiningstep.

FIG. 10 is a side view of a single lamina following a third machiningstep.

FIG. 11 is a top view of a single lamina following a third machiningstep.

FIG. 12 is a perspective view of a single lamina following a thirdmachining step.

FIG. 13 is a perspective view of a plurality of laminae suitable for usein the disclosed methods.

FIG. 14 is an end view of the plurality of laminae oriented in a firstorientation.

FIG. 15 is an end view of the plurality of laminae following a firstmachining operation.

FIG. 16 is a side view of the plurality of laminae following a firstmachining operation.

FIG. 17 is an end view of the plurality of laminae oriented in a secondorientation.

FIG. 18 is an end view of the plurality of laminae following a secondmachining operation.

FIG. 19 is a side view of the plurality of laminae following a secondmachining operation.

FIG. 20 is an side view of the plurality of laminae following a thirdmachining operation.

FIG. 21 is a top view of the plurality of laminae following a thirdmachining operation.

FIG. 22 is an end view of a single lamina after a first machiningoperation according to an alternate embodiment.

FIG. 23 is a side elevation view of the embodiment depicted in FIG. 22.

FIG. 24 is a top plan view of the lamina depicted in FIG. 22.

FIG. 25 is an end view of a single lamina after a second machiningoperation according to an alternate embodiment.

FIG. 26 is a side elevation view of the embodiment depicted in FIG. 25.

FIG. 27 is a top plan view of the lamina depicted in FIG. 25.

FIG. 28 is a side elevation view of a single lamina after a secondmachining operation according to an alternate embodiment.

FIG. 29 is an end view of the embodiment depicted in FIG. 28.

FIG. 30 is a top plan view of the lamina depicted in FIG. 28.

FIG. 31 is a top plan view of a portion of the working surface of asingle lamina.

FIG. 32 is a side elevation view of the working surface depicted in FIG.31.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In describing various embodiments, specific terminology will be used forthe sake of clarity. Such terminology is not, however, intended to belimiting and it is to be understood that each term so selected includesall technical equivalents that function similarly. Related applicationsfiled on Jul. 2, 1997 include: Cube Comer Sheeting Mold and Method ofMaking the Same (U.S. Ser. No. 08/886,074); Retroreflective Cube CornerSheeting, Molds Therefore, and Methods of Making the Same (U.S. Ser. No.08/887,390); Tiled Retroreflective Sheeting Composed of Highly CantedCube Comer Elements (U.S. Ser. No. 08/887,389, now issued as U.S. Pat.No. 5,898,523); Retroreflective Cube Comer Sheeting Mold and Method forMaking the Same (U.S. Ser. No. 08/887,074); and Dual OrientationRetroreflective Sheeting (U.S. Ser. No. 08/887,006, now issued as U.S.Pat. No. 5,936,770).

The disclosed embodiments can utilize full cube corner elements of avariety of sizes and shapes. The base edges of adjacent full cube cornerelements in an array are not all in the same plane. By contrast, thebase edges of adjacent truncated cube corner elements in an array aretypically coplanar. Full cube corner elements have a higher total lightreturn than truncated cube corner elements for a given amount of cant,but the full cubes lose total light return more rapidly at higherentrance angles. One benefit of full cube corner elements is highertotal light return at low entrance angles, without too large of a lossin performance at higher entrance angles.

Predicted total light return (TLR) for a cube corner matched pair arraycan be calculated from a knowledge of percent active area and rayintensity. Ray intensity may be reduced by front surface losses and byreflection from each of the three cube corner surfaces for aretroreflected ray. Total light return is defined as the product ofpercent active area and ray intensity, or a percentage of the totalincident light which is retroreflected. A discussion of total lightreturn for directly machined cube corner arrays is presented in U.S.Pat. No. 3,712,706 (Stamm).

One embodiment of a lamina, as well as a method of making the same, willnow be described with reference to FIGS. 1-12. Referring to FIG. 1,there is illustrated a representative lamina 10 useful in themanufacture of a mold suitable for forming retroreflective sheeting.Lamina 10 includes a first major surface 12 and an opposing second majorsurface 14. Lamina 10 further includes a working surface 16 and anopposing bottom surface 18 extending between first major surface 12 andsecond major surface 14. Lamina 10 further includes a first end surface20 and an opposing second end surface 22. In a preferred embodiment,lamina 10 is a right rectangular polyhedron wherein opposing surfacesare substantially parallel. However, it will be appreciated thatopposing surfaces of lamina 10 need not be parallel.

For purposes of description, a Cartesian coordinate system can besuperimposed onto lamina 10. A first reference plane 24 is centeredbetween first major surface 12 and second major surface 14. Firstreference plane 24, referred to as the x-z plane, has the y-axis as itsnormal vector. A second reference plane 26, referred to as the x-yplane, extends substantially co-planar with working surface 16 of lamina10 and has the z-axis as its normal vector. A third reference plane 28,referred to as the y-z plane, is centered between first end surface 20and second end surface 22 and has the x-axis as its normal vector. Forthe sake of clarity, various geometric attributes of the presentinvention will be described with reference to the Cartesian referenceplanes as set forth herein. However, it will be appreciated that suchattributes can be described using other coordinate systems or withreference to the structure of the lamina.

The laminae are preferably formed from a dimensionally stable materialcapable of holding precision tolerances, such as machinable plastics(for example, polyethylene teraphthalate, polymethyl methacrylate, andpolycarbonate) or metals (for example, brass, nickel, copper, oraluminum). The physical dimensions of the laminae are constrainedprimarily by machining limitations. The laminae preferably measure atleast 0.1 millimeters in thickness, between 5.0 and 100.0 millimeters inheight, and between 10 and 500 millimeters in width. These measurementsare provided for illustrative purposes only and are not intended to belimiting. By way of example, the thickness of each lamina can measurebetween about 0.025 and about 5.00 millimeters, between about 0.025 andabout 1.00 millimeters, between about 0.1 and about 1.00 millimeters, orbetween about 0.1 to about 0.6 millimeters. FIGS. 2-12 illustrate theformation of a structured surface comprising a plurality of opticallyopposing cube corner elements in the working surface 16 of lamina 10. Inbrief summary, a first groove set comprising a plurality of parallel,adjacent grooves 30 a, 30 b, 30 c, etc. (collectively referred to as 30)is formed in the working surface 16 of lamina 10 (FIGS. 2-4). Thegrooves 30 define first groove surfaces 32 a, 32 b, 32 c, etc. andsecond groove surfaces 34 b, 34 c, 34 d, etc. A second groove setcomprising at least one, and preferably a plurality of parallel,adjacent grooves 38 a, 38 b, 38 c, etc. (collectively referred to as 38)is also formed in the working surface 16 of lamina 10 (FIGS. 5-7). Thegrooves 38 define third groove surfaces 40 a, 40 b, 40 c, etc. andfourth groove surfaces 42 b, 42 c, 42 d, etc. Importantly, the firstgroove surfaces 32 a, 32 b, 32 c, etc. intersect the respective thirdgroove surfaces 40 a, 40 b, 40 c, etc. substantially orthogonally todefine respective first reference edges 44 a, 44 b, 44 c, etc., and thesecond groove surfaces 34 b, 34 c, 34 d, etc. intersect the respectivefourth groove surfaces 42 b, 42 c, 42 d, etc. substantially orthogonallyto define respective second reference edges 45 b, 45 c, 45 d, etc. Asused herein, the terms ‘substantially orthogonally’ or ‘approximatelyorthogonally’ shall mean that the dihedral angle between the respectivesurfaces measures approximately 90°; slight variations in orthogonalityas disclosed and claimed in U.S. Pat. No. 4,775,219 to Appeldorn arecontemplated. A third groove set comprising a plurality of paralleladjacent grooves 46 a, 46 b, 46 c, etc. is then formed in the workingsurface 16 of lamina 10 (FIGS. 9-11). The grooves of the third grooveset define respective fifth groove surfaces 48 a, 48 b, 48 c, etc. andsixth groove surfaces 50 a, 50 b, 50 c, etc. Importantly, the respectivefifth groove surfaces 48 a, 48 b, 48 c, etc. intersect the respectivefirst groove surfaces 32 a, 32 b, 32 c, etc. and third groove surfaces40 a, 40 b, 40 c, etc. substantially orthogonally to form a plurality ofcube corner elements. Additionally, the respective sixth groove surfaces50 a, 50 b, 50 c, etc. intersect the respective second groove surfaces34 b, 34 c, 34 d, etc. and fourth groove surfaces 42 b, 42 c, 42 d, etc.substantially orthogonally to form a plurality of cube corner elements.As used herein, the term ‘groove set’ refers to a plurality of parallel,although not necessarily coplanar, grooves formed in working surface 16of the lamina 10.

Referring now to FIGS. 2-4, a first groove set comprising at least one,and preferably a plurality of parallel, adjacent grooves 30 a, 30 b, 30c, etc. (collectively referred to as 30) is formed in the workingsurface 16 of lamina 10. The grooves define first groove surfaces 32 a,32 b, 32 c, etc. (collectively referred to as 32) and second groovesurfaces 34 b, 34 c, 34 d, etc. (collectively referred to as 34) thatintersect at groove vertices 33 a, 33 b, 33 c, 33 d, etc. (collectivelyreferred to as 33) and along edges 36 a, 36 b, 36 c, etc., as shown. Atthe edge of the lamina, the groove forming operation may form a singlegroove surface, e.g. 32 a, 34 d. Preferably this pattern is repeatedacross the entire working surface 16 of lamina 10 as illustrated inFIGS. 2-4. The groove vertices 33 are preferably separated by a distancethat measures between about 0.01 millimeters and about 1.0 millimeters,however it is not intended that the present invention be limited bythese dimensions

Referring particularly to FIG. 2, the grooves 30 are formed such thatthe respective groove vertices 33 extend along an axis that intersectsfirst major surface 12, second major surface 14, and second referenceplane 26. In the embodiment depicted in FIGS. 2-4 the grooves 30 areformed such that each of the respective groove vertices 33 are disposedin planes that intersect the first reference plane 24 and the secondreference plane 26 at orthogonal angles such that, in the top view ofFIG. 4, the respective groove vertices 33 appear perpendicular to firstreference plane 24.

In FIGS. 2-4, the respective groove vertices 33 intersect the secondreference plane 26 at an acute angle θ₁ that measures approximately54.74°. It will be appreciated, however, that grooves 30 may be formedsuch that the respective groove vertices 33 intersect second referenceplane 26 at angles different than 54.74°. In general, it is feasible toform grooves such that the respective groove vertices 33 intersect thesecond reference planes at any angle between about 45° and nearly 90°.Additionally, the dihedral angle between opposing faces of grooves (e.g.34 b and 32 b), measures 120° in the embodiment depicted in FIGS. 2-4.More generally, this angle can vary between 90° and 180°.

Referring now to FIGS. 5-8, a second groove set comprising at least twoparallel, adjacent grooves 38 a, 38 b, 38 c, etc. (collectively referredto as 38) is formed in the working surface 16 of lamina 10. The groovesdefine third groove surfaces 40 a, 40 b, 40 c, etc. (collectivelyreferred to as 40) and fourth groove surfaces 42 b, 42 c, 42 d, etc.(collectively referred to as 42) that intersect at a groove vertex 41 b,41 c, 41 d, etc. (collectively referred to as 41) and edges 47 a, 47 b,47c, etc. At the edge of the lamina, the groove forming operation mayform a single groove surface, e.g. 40 a, 42 d. Groove 38 a is formedsuch that groove surfaces 32 a and 40 a intersect approximatelyorthogonally along a first reference edge 44 a. Similarly, groove 38 bis formed such that groove surfaces 34 b and 42 b intersectapproximately orthogonally along a second reference edge 45 b and groovesurfaces 32 b and 40 b intersect approximately orthogonally along areference edge 44 b. Preferably this pattern is repeated across theentire working surface 16 of lamina 10. The respective groove vertices41 are preferably separated by a distance that measures between about0.01 millimeters and about 1.0 millimeters, however it is not intendedthat the present invention be limited by these dimensions.

Referring again to FIGS. 5-8, grooves 38 are formed such that therespective groove vertices 41 extend along an axis that intersectssecond major surface 14 and second reference plane 26. Similarly, thegrooves 38 are formed such that each of the respective groove vertices41 are disposed in planes that intersect the first reference plane 24and the second reference plane 26 at orthogonal angles such that, in thetop view of FIG. 7, the respective groove vertices 41 appearperpendicular to first reference plane 24. Additionally, referringparticularly to FIG. 7, it can be seen that the grooves 38 in the secondgroove set are preferably formed such that the respective groovevertices 41 a, 41 b, 41 c, etc. are substantially coplanar withrespective groove vertices 33 a, 33 b, 33 c of first groove set 30. Itwill be appreciated, however, that opposing respective groovevertices(e.g. 33, 41) need not be coplanar.

The respective groove vertices 41 intersect the second reference plane26 at an acute angle θ₂ that measures approximately 54.74°. It will beappreciated, however, that grooves 38 may be formed such that therespective groove vertices 41 intersect second reference plane 26 atangles different than 54.74°. Additionally, although the disclosedembodiment is manufactured such that θ₁ is equal to θ₂, these angles maydiffer from one another. The relationship between angles θ₁ and θ₂ isdiscussed in greater detail below. In general, it is feasible to formgrooves such that the respective groove vertices 41 intersect the secondreference plane 26 at any angle between about 45° and about 90°, morepreferably, however, the grooves are formed such that the angle θ₁ isequal to θ₂, and the angles preferably measure between about 45° andabout 60°. In the disclosed embodiment the dihedral angle betweenopposing faces of grooves 38 (e.g. 42 b and 40 b), measures 120°. Thus,reference edges 44, 45 are disposed at angles γ₁ and γ₂, respectively,that measure approximately 45° from second reference plane 26.

FIG. 8 presents a perspective view of a representative lamina 10 uponcompletion of forming the grooves 38 in the second groove set. Lamina 10includes a series of grooves 30, 38 formed in the working surface 16thereof as described above. The respective groove vertices intersectapproximately along the first reference plane 24 to define a pluralityof substantially V-shaped valleys in the working surface 16 of lamina10.

FIGS. 9-12 illustrate an embodiment of lamina 10 following formation ofa third groove set comprising a plurality of groovs 46 a, 46 b, 46 c,etc. in lamina 10. In the disclosed embodiment the third grooves 46define respective fifth groove surfaces 48 a, 48 b, 48 c, etc. andrespective sixth groove surfaces 50 a, 50 b, 50 c, etc. that intersectat respective groove vertices 52 a, 52 b, 52 c. The grooves 46 areformed such that the respective groove vertices 52 extend along an axisthat is substantially perpendicular to first reference plane 24. Thethird grooves 46 are formed such that the respective fifth groovesurfaces 48 are disposed in planes that are substantially orthogonal tothe respective first groove surfaces 32 and the respective third groovesurfaces 40 and the respective sixth groove surfaces 50 are disposed inplanes that are substantially orthogonal to the respective second groovesurfaces 34 and the respective fourth groove surfaces 42. In thedisclosed embodiment third grooves 46 are formed such that therespective groove surfaces 48, 50 are disposed at angles α₁, α₂,respectively, that measure 45° from an axis 82 normal to secondreference plane 26. More generally, the angle α₁ is equal to γ₁ and theangle α₂ is equal to γ₂.

Formation of the respective fifth groove surfaces 48 according to theinvention yields a plurality of cube corner elements 60 a, 60 b, etc.(collectively referred to by reference numeral 60) in working surface 16of lamina 10 having three mutually perpendicular optical surfaces. Eachcube corner element 60 is defined by a respective first groove surface32 a, 32 b, 32 c, etc., a respective third groove surface 40 a, 40 b, 40c, etc. and a respective fifth groove surface 48 a, 48 b, 48 c, etc.that mutually intersect at a point to define a respective cube cornerpeak, or apex 62 a, 62 b, 62 c, etc. Similarly, formation of therespective sixth groove surfaces 50 also yields a plurality of cubecorner elements 70 a, 70 b, 70 c, etc. (collectively referred to byreference numeral 70) in working surface 16 of lamina 10. Each cubecorner element 70 is defined by a respective second groove surface 34 b,34 c, 34 d, etc., a respective fourth groove surface 42 b, 42 c, 42 d,etc. and a respective sixth groove surface 50 a, 50 b, 50 c, etc. thatmutually intersect at a point to define a respective cube corner peak,or apex 63 a, 63 b, 63 c, etc. Preferably, both fifth groove surface 48and sixth groove surface 50 form a plurality of cube corner elements onthe working surface 16 of lamina 10. However, it will be appreciatedthat in alternate embodiments the respective third grooves 46 could beformed such that only the fifth groove surfaces 48 or the sixth groovesurfaces 50 form cube corner elements.

Preferably, working surface 16 is formed using conventional precisionmachining tooling and techniques. Appropriate material removaltechniques for forming the grooves in lamina 10 include precisionengineering techniques such as, for example, ruling, milling, grooving,and fly-cutting. In one embodiment second major surface 14 of lamina 10can be registered to a substantially planar surface such as the surfaceof a precision machining fixture and each groove 30 a, 30 b, 30 c, etc.can be formed in working surface 16 by moving a V-shaped cutting toolhaving an included angle of 120° along an axis that intersects the firstworking surface 12 and the first reference plane 24 at an angle of about35.26° (90°−θ₁).

In the disclosed embodiment each respective groove 30 is formed at thesame depth in working surface 16 and the cutting tool is moved laterallyby the same distance between adjacent grooves such that grooves aresubstantially identical. Next, first major surface 12 of lamina 10 canbe registered to the planar surface and each groove 38 a, 38 b, 38 c,etc. can be formed in working surface 16 by moving a V-shaped cuttingtool having an included angle of 120° along an axis that intersects thesecond working surface 14 and the first reference plane 24 at an angleof about 35.26° (90°−θ₁). Finally, third grooves 46 a, 46 b, 46 c, etc.can be formed in working surface 16 by moving a V-shaped cutting toolhaving an included angle of 90° along an axis substantiallyperpendicular to first reference plane 24.

While the three groove forming steps have been recited in a particularorder, one of ordinary skill in the art will recognize that the order ofthe steps is not critical; the steps can be practiced in any order.Additionally, one of ordinary skill in the art will recognize that thethree groove sets can be formed with the lamina registered in oneposition; the present disclosure contemplates such a method.Furthermore, the particular mechanism for securing the lamina to theprecision machining fixture is not critical; physical, chemical, andelectromagnetic mechanisms of securing the lamina can be used.

To form a mold suitable for use in forming retroreflective articles, aplurality of laminae 10 having a working surface 16 that includes cubecorner elements 60, 70 formed as described above can be assembledtogether in a suitable conventional fixture. Working surface 16 can thenbe replicated using precision replication techniques such as, forexample, nickel electroplating to form a negative copy of workingsurface 16. Electroplating techniques are known to those of ordinaryskill in the retroreflective arts. See e.g. U.S. Pat. Nos. 4,478,769 and5,156,863 to Pricone et al. The negative copy of working surface 16 canthen be used as a mold for forming retroreflective articles having apositive copy of working surface 16. More commonly, additionalgenerations of electroformed replicas are formed and assembled togetherinto a larger mold. It will be noted that the original working surfaces16 of the lamina 10, or positive copies thereof, could also be used asan embossing tool to form retroreflective articles. See, JP 8-309851 andU.S. Pat. No. 4,601,861 (Pricone). One of ordinary skill in theretroreflective arts will recognize that the working surface 16 of eachlamina 10 functions independently as a retroreflector. Thus, adjacentlamina in the mold need not be positioned at precise angles or distancesrelative to one another.

FIGS. 13-21 present another method for forming a plurality of laminaesuitable for use in a mold suitable for use in forming retroreflectivearticles. In the embodiment depicted in FIGS. 13-21, a plurality of cubecorner elements are formed in the working surfaces of a plurality oflaminae while the laminae are secured in an assembly, rather thanindependently, as described above. The plurality of laminae 10 arepreferably assembled such that their respective working surfaces 16 aresubstantially co-planar. In brief summary, the plurality of laminae 10are oriented such that their respective major planes are disposed at afirst angle, θ₁, relative to a fixed reference axis 82 (see FIG. 14). Afirst groove set preferably comprising a plurality of parallel, adjacentV-shaped grooves is formed in the working surface 16 of the plurality oflaminae 10 (FIGS. 15-16). The plurality of laminae are then orientedsuch that their respective major planes are disposed at a second angle,θ₂, relative to the reference axis 82 (see FIG. 17). A second groove setcomprising a plurality of parallel, adjacent V-shaped grooves is formedthe working surface 16 of the plurality of laminae 10 (FIGS. 18-19). Theplurality of laminae are then oriented such that their respective firstreference planes are disposed substantially parallel to the referenceaxis and a third groove set comprising a plurality of V-shaped groovesin the working surface 16 of each lamina 10 is formed (FIG. 20).Formation of the third groove set results in a structured surface thatincludes a plurality of cube corner elements on the working surface ofthe plurality of laminae 10 (FIG. 21).

The embodiment illustrated in FIGS. 13-21 will now be described ingreater detail. Referring to FIG. 13, there is illustrated a pluralityof thin laminae 10 assembled together such that the first major surface12 of one lamina 10 is adjacent the second major surface 14 of anadjacent lamina 10. Preferably, the plurality of laminae 10 areassembled in a conventional fixture capable of securing the plurality oflaminae adjacent one another. Details of the fixture are not critical.For purposes of description, however, the fixture preferably defines abase plane 80 which, in a preferred embodiment, is substantiallyparallel to the bottom surfaces 18 of the respective lamina 10 when thelamina 10 are positioned as depicted in FIG. 13. The plurality oflaminae 10 can be characterized in three dimensional space by aCartesian coordinate system as described above. Preferably, therespective working surfaces 16 of the plurality of laminae 10 aresubstantially coplanar when the lamina are positioned with theirrespective first reference planes 24 perpendicular to base plane 80.

Referring to FIG. 14, the plurality of laminae 10 are oriented to havetheir respective first reference planes 24 disposed at a first angle,θ₁, from a fixed reference axis 82 normal to base plane 80. In oneembodiment, the angle θ₁ measures approximately 54.74°. In theory, theangle θ₁ can be any angle between about 45° and about 90°, however, inpractice the angle θ₁ can typically measure between about 45° and about60°. Referring to FIGS. 15-16, a first groove set comprising a pluralityof parallel adjacent V-shaped grooves 30 a, 30 b, 30 c, etc.(collectively referred to by reference numeral 30) is formed in theworking surfaces 16 of the plurality of laminae 10 with the laminadisposed at angle θ₁. The grooves 30 define respective first groovesurfaces 32 a, 32 b, 32 c, etc. (collectively referred to by thereference numeral 32) and respective second groove surfaces 34 b, 34 c,34 d, etc. (collectively referred to by the reference numeral 34) thatintersect at respective groove vertices 33 b, 33 c, 33 d, etc.(collectively referred to by the reference numeral 33). It will be notedthat, at the edge of the lamina, the groove forming operation may form asingle groove surface, e.g. 32 b, 34 d. Preferably this pattern isrepeated across the entire working surfaces 16 of the plurality oflaminae 10.

The grooves 30 are formed by removing portions of working surface 16 ofthe plurality of laminae using any one of a wide variety of materialremoval techniques including precision machining techniques such asmilling, ruling, and fly-cutting, or chemical etching or laser ablationtechniques. According to one embodiment, the grooves 30 of the firstgroove set are formed in a high-precision machining operation in which adiamond cutting tool having a 120° included angle is repeatedly movedtransversely across the working surfaces 16 of the plurality of laminae10 along an axis that is substantially parallel to base plane 80. Itwill be appreciated, however that the diamond cutting tool could bemoved along an axis that is non-parallel to base plane 80 such that thetool cuts at a varying depth across the plurality of laminae 10. It willalso be appreciated that the machining tool could be held stationarywhile the plurality of laminae are placed in motion; the presentdisclosure contemplates relative motion between the plurality of laminae10 and the machining tool.

In the embodiment depicted in FIGS. 15-16, the grooves 30 of the firstgroove set are formed at a depth such that the respective groovevertices 33 intersect the first major surface 12 and the second majorsurface 14 of each lamina. Thus, in the end view depicted in FIG. 15,groove vertices 33 form substantially continuous lines that extend alongan axis parallel to base plane 80. Further, grooves 30 are formed suchthat the groove vertices 33 and the edges 36 are disposed in planes thatintersect the respective first reference planes 24 and the secondreference plane 26 at orthogonal angles. Thus, in a top plan viewanalogous to FIG. 4, the respective groove vertices would appearperpendicular to the respective first reference planes 24 of theplurality of laminae 10. However, grooves 30 can alternately be formedat lesser depths or along different axes.

Referring to FIGS. 17-19, the plurality of laminae 10 are then orientedto have their respective first reference planes 24 disposed at a secondangle, θ₂, from fixed reference axis 82 normal to base plane 80 and asecond groove set comprising a plurality of parallel adjacent V-shapedgrooves 38 b, 38 c, etc. (collectively referred to by reference numeral38) is formed in the working surfaces 16 of the plurality of laminae 10.In the disclosed embodiment, the angle θ₂ measures approximately 54.74°.As discussed above, in theory, the angle θ₂ can be any angle between 45°and 90°, however, in practice the angle θ₂ preferably measures betweenapproximately 45° and 60°. To orient the plurality of laminae 10 atangle θ₂, the laminae 10 are preferably removed from the fixture andreassembled with their respective first reference planes disposed atangle θ₂. The grooves 38 define respective third groove surfaces 40 a,40 b, 40 c, etc. (collectively referred to by the reference numeral 40)and respective fourth groove surfaces 42 b, 42 c, 42 d, etc.(collectively referred to by the reference numeral 42) that intersect atrespective groove vertices 41 b, 41 c, 41 d, etc. (collectively referredto by the reference numeral 41) and along edges 47 a, 47 b, 47c, etc. Itwill be noted that, at the edge of the lamina, the groove formingoperation may form a single groove surface, e.g. 40 a, 42 d. Preferablythis pattern is repeated across the entire working surfaces 16 of theplurality of laminae 10.

Grooves 38 of the second groove set are also preferably formed by ahigh-precision machining operation in which a diamond cutting toolhaving a 120° included angle is repeatedly moved transversely across theworking surfaces 16 of the plurality of laminae 10 along a cutting axisthat is substantially parallel to base plane 80. Grooves 38 arepreferably formed at approximately the same depth in working surface 16of the plurality of laminae 10 as grooves 30 in first groove set.Additionally, the grooves 38 in the second groove set are preferablyformed such that the respective groove vertices (e.g. 41 a, 41 b, etc.)are substantially coplanar with respective groove vertices (e.g. 33 a,33 b, etc.) of the grooves 30 in the first groove set. After forming thegrooves 38 in the second groove set, each lamina 10 preferably appearssubstantially identical to the lamina presented in FIG. 8.

Referring to FIGS. 20-21, a third groove set comprising a plurality ofparallel adjacent V-shaped grooves 46 a, 46 b, 46 c etc. (collectivelyreferred to by reference numeral 46) is formed in the working surfaces16 of the plurality of laminae 10. The third grooves 46 a, 46 b, 46 c,etc. (collectively referred to as 46) define respective fifth groovesurfaces 48 a, 48 b, 48 c, etc. (collectively referred to as 48) andrespective sixth groove surfaces 50 a, 50 b, 50 c, etc. (collectivelyreferred to as 50) that intersect at a respective groove vertices 52 a,52 b, 52 c, etc. (collectively referred to as 52). Significantly, therespective third grooves 46 are formed such that respective fifth groovesurfaces (e.g. 48 a, 48 b, 48 c, etc.) are disposed substantiallyorthogonal to the respective first groove surfaces (e.g. 32 a, 32 b,etc.) and the respective third groove surfaces (e.g. 40 a, 40 b, 40 c,etc.).

Formation of the fifth groove surfaces 48 as described yields aplurality of cube corner elements (e.g. 60 a, 60 b, 60 c, etc.),collectively referred to by reference numeral 60, in working surface 16of the respective lamina 10. Each cube corner element 60 is defined by afirst groove surface 32 a third groove surface 40 and a fifth groovesurface 48 that mutually intersect at a point to define a cube cornerpeak, or apex 62. Similarly, the respective sixth groove surfaces (e.g.50 a, 50 b, 50 c, etc.) are disposed substantially orthogonal to therespective second groove surfaces (e.g. 34 a, 34 b, 34 c, etc.) and therespective fourth groove surfaces (e.g. 42 a, 42 b, 42 c, etc.).Formation of the sixth groove surfaces 50 also yields a plurality ofcube corner elements 70 a, 70 b, etc. (collectively referred to byreference numeral 70) in working surface 16 of lamina 10. Each cubecorner element 70 is defined by a second groove surface 34, a fourthgroove surface 42 and a sixth groove surface 50 that mutually intersectat a point to define a cube corner peak, or apex 72. Preferably, bothfifth groove surface 48 and sixth groove surface 50 form a plurality ofoptically opposing cube corner elements on the working surface 16 oflamina 10. However, it will be appreciated that third groove 46 could beformed such that only fifth groove surfaces 48 or sixth groove surfaces50 form cube corner elements.

An array of cube corner elements 60, 70 each having three mutuallyperpendicular optical faces 32, 40, 48 and 34, 42, 50, respectively, arepreferably formed on a single lamina. All three optical faces arepreferably formed by the machining process to ensure optical qualitysurfaces. A planar interface 12, 14 is preferably maintained betweenadjacent laminae during the machining phase and subsequent thereto so asto minimize alignment problems and damage due to handling of thelaminae.

In a preferred method, the plurality of laminae 10 are re-oriented tohave their respective major planes 24 disposed approximately parallel toreference axis 82 before forming the plurality of grooves 46. In apreferred embodiment a diamond cutting tool having an included angle of90° is moved across the working surfaces 16 of the plurality of laminae10 along an axis that is substantially parallel to base plane 80.However, the grooves 46 can be formed with the lamina oriented such thattheir respective major planes are disposed at an angle relative toreference axis 82. Grooves 46 are preferably formed such that therespective groove vertices 52 are slightly deeper than the vertices ofthe grooves in the first and second groove sets. Formation of grooves 46result in a plurality of laminae 10 having a structured surfacesubstantially as depicted in FIG. 12.

Working surface 16 exhibits several desirable characteristics as aretroreflector. The cube corner element geometry formed in workingsurface 16 of lamina 10 can be characterized as a ‘full’ or ‘highefficiency’ cube corner element geometry because the geometry exhibits amaximum effective aperture that approaches 100% provided the cube cornerpeaks are positioned approximately in the center of the cube cornerelement. It will be recognized by one of ordinary skill in theretroreflective arts that the cube corner elements can be designed withtheir respective peaks offset from the center to address wide entranceangle performance issues or other issues. Thus, a retroreflector formedas a replica of working surface 16 will exhibit high optical efficiencyin response to light incident on the retroreflector approximately alongthe symmetry axes of the cube corner elements. Additionally, cube cornerelements 60 and 70 are disposed in opposing orientations and aresymmetrical with respect to first reference plane 24 and will exhibitsymmetric retroreflective performance in response to light incident onthe retroreflector at high entrance angles.

FIGS. 22-30 illustrate an alternate embodiment in which a single laminais provided with a plurality of cube corner elements that are notoptically opposing in orientation. Rather, the cube corner elementsdepicted in FIGS. 22-30 are disposed in substantially the sameorientation. Thus, a retroreflective sheeting formed as a replica of thelamina presented in FIGS. 22-30 will exhibit highly asymmetricalentrance angularity performance. This may be desirable forunidirectional retroreflection applications such as, for example,barricade markers or certain pavement marking applications. A method offorming such a lamina is illustrated particularly with reference to asingle lamina. However, it will be appreciated that the machiningtechniques disclosed in connection with FIGS. 13-21 are equallyeffective to produce a plurality of lamina.

In brief summary, a first groove set comprising a plurality of parallel,adjacent grooves 130 a, 130 b, 130 c, etc. (collectively referred to bythe reference numeral 130) is formed in the working surface 116 oflamina 110 (FIGS. 22-24). The grooves of the first groove set definerespective first groove surfaces 132 a, 132 b, 132 c, etc. andrespective second groove surfaces 134 a, 134 b, 134 c, etc. A secondgroove set comprising at least one, and preferably a plurality ofparallel, adjacent grooves 138 a, 138 b, 138 c, etc. (collectivelyreferred to by the reference numeral 138) is also formed in the workingsurface 116 of lamina 110 (FIGS. 25-27). The grooves of the secondgroove set define respective third groove surfaces 140 a, 140 b, 140 c,etc. and fourth groove surfaces 142 a, 142 b, 142 c, etc. Importantly,the respective first groove surfaces 132 a, 132 b, 132 c, etc. intersectthe respective third groove surfaces 140 a, 140 b, 140 c, etc.substantially orthogonally to define respective first reference edges144 a, 144 b, 144 c, 144 d, etc. In the disclosed embodiment therespective second groove surfaces 134 b, 134 c, 134 d, etc. aresubstantially coplanar with the respective fourth groove surfaces 142 b,142 c, 142 d, etc. A third groove set comprising a plurality of paralleladjacent grooves 146 a, 146 b, 146 c, etc. is then formed in the workingsurface 116 of lamina 110 (FIGS. 28-30). The grooves of the third grooveset define respective fifth groove surfaces 150 a, 150 b, 150 c, etc.that intersect the respective first groove surfaces 132 a, 132 b, 132 c,etc. and third groove surfaces 140 a, 140 b, 140 c, etc. at an apex 162a, 162 b, 162 c, 162 d, etc. substantially orthogonally to form aplurality of cube corner elements 160 a, 160 b, 160 c, disposed in thesame orientation on lamina 110.

The lamina depicted in FIGS. 21-30 is preferably formed using precisionmachining techniques as described above. One embodiment of a lamina maybe manufactured by machining the first groove set 130 using a cuttingtool that is asymmetric about its vertical axis and having an includedangle that measures approximately 66.1° along an axis that intersectssecond reference plane 26 at an angle θ₁ that measures approximately50.7°. Similarly, second groove set 138 is preferably formed bymachining with a cutting tool that is asymmetric about its vertical axisand having an included angle that measures approximately 66.1° along anaxis that intersects second reference plane 26 at an angle θ₂ thatmeasures approximately 50.7°. Finally, third groove set 146 ispreferably formed by machining with a half-angle tool having an includedangle α that measures approximately 35° along an axis substantiallyperpendicular to first reference plane 24. The edges 144 a, 144 b, 144c, 144 d, etc. are disposed at an angle γ₁, respectively, that measureapproximately 35° from second reference plane 26. In the embodiment ofFIG. 28, α=γ₁.

The foregoing discussion has disclosed several particular embodiments ofcube corner element geometries and the associated machiningconfigurations required to produce the geometries. Methods of thepresent disclosure can be utilized to produce a wide variety of cubecorner element geometries by altering the groove angles, (e.g. α₁, α₂),and the angle at which the laminae are tilted (e.g. θ₁and θ₂) to therebychange the orientation of the cube corner elements on the workingsurface of the laminae. Further contemplated are articles manufacturedas replicas of the laminae. The preceding discussion disclosed severalembodiments of cube corner geometries. The following paragraphs providea generic description of the angular relationships between the faces ofthe cube corner elements such that one of ordinary skill in the artcould produce a wide variety of cube corner element geometries.

FIGS. 31-32 present a top plan view and side elevation views of theworking surface of a lamina 410 that has a single cube corner element460 formed therein. Lamina 410 may be characterized in 3-dimensionalspace by first, second and third reference planes 424, 426 and 428,respectively. For purposes of illustration, cube corner element 460 maybe defined as a unit cube consisting of three substantially mutuallyperpendicular optical faces 432, 434, 448. Optical face 432 is formed byone optical surface of a first groove 430 formed in the working surfaceof lamina 410 and optical face 434 is formed by an optical surface of asecond groove 438 formed in the working surface of lamina 410. Opticalface 448 is formed by one surface of groove 446. Reference plane 456 ais parallel to the vertex of groove 446 and perpendicular to secondreference plane 426. Similarly, reference plane 456 b is parallel to thevertex of groove 446 and perpendicular to the second reference plane.Reference planes 456 a and 456 b are disposed at an angle φ₃ relative tothird reference plane 428. The angle φ₃ corresponds to the degree ofangular rotation of the cube corner element on the surface of thelamina. Subject to machining limitations, the angle φ₃ can range from0°, such that the groove sets are formed along axes substantiallycoincident with reference planes 424 and 428, to nearly 90°. Preferably,however the angle φ₃ measures between 0° and 45°.

Optical face 448 is disposed at an angle α₁ from reference plane 456 a.Similarly Optical face 432 is disposed at an angle α₂ from referenceplane 456 b and optical face 434 is disposed at an angle α₃ fromreference plane 456 b. Preferably, unit cube 460 is formed usingconventional precision machining techniques and angles α₁, α₂ and α₃correspond to the included angles of the cutting tools used to form thegrooves that define cube corner element 460.

FIG. 32 presents a side elevation view of unit cube 460 taken alonglines 31—31. The vertex 433 of groove 430 is disposed at an acute angleβ₁ relative to second reference plane 426. Similarly, the vertex 441 ofgroove 438 is disposed at an acute angle β₂ relative to second referenceplane 426. The orientation in space of optical face 432 is a function ofthe groove angle α₁ and of angle β₁. Similarly, the orientation in spaceof optical face 434 is a function of the groove angle α₂ and of angleβ₂.

A second Cartesian coordinate system can be established using the groovevertices that form unit cube 460 as reference axes. In particular, thex-axis 472 can be established parallel to the intersection of plane 456a and second reference plane 426, the y-axis 474 can be establishedparallel to the second reference plane 426 and perpendicular to thex-axis, and the z-axis 476 extends perpendicular to second referenceplane 426. Adopting this coordinate system, unit normal vectors N₁, N₂and N₃ can be defined for the unit cube surfaces 448, 432, and 434,respectively as follows:

N ₁=cos(α₁)j+sin(α₁)k

N ₂=sin(α₂)sin(β3 ₁)i−cos(α₂)j+cos(β₁)sin(α₂)k

N ₃=−sin(β₂)sin(α₃)i−cos(α₃)j+cos(β₂)sin(α₃)k

Surfaces 432, 434 and 448 must be substantially mutually perpendicular.Thus, the dot products of the normal vectors equal zero.

N ₁ ·N ₂ =N ₂ ·N ₃ =N ₁ ·N ₃=0.

Therefore, the following conditions hold:

tan(α₁)tan(α₂)cos(β₁)=1

tan(α₁)tan(α₂)cos(β₂)=1

tan(β₁)tan(β₂)=1+tan²(α₁).

These equations define the geometric constraints specifically for unitcube 460. The general approach can be applied by one knowledgeable inthe cube corner arts with differing orientations including, for example,cube corner 460.

In the manufacture of retroreflective articles such as retroreflectivesheeting, the structured surface of the plurality of laminae is used asa master mold which can be replicated using electroforming techniques orother conventional replicating technology. The plurality of laminae caninclude substantially identical cube corner elements or can include cubecorner elements of varying sizes, geometries, or orientations. Thestructured surface of the replica, referred to in the art as a ‘stamper’contains a negative image of the cube corner elements. This replica canbe used as a mold for forming a retroreflector. More commonly, however,a large number of positive or negative replicas are assembled to form amold large enough to be useful in forming retroreflective sheeting.Retroreflective sheeting can then be manufactured as an integralmaterial, e.g. by embossing a preformed sheet with an array of cubecorner elements as described above or by casting a fluid material into amold. Alternatively, the retroreflective sheeting can be manufactured asa layered product by casting the cube corner elements against apreformed film as taught in PCT application No. WO 95/11464 and U.S.Pat. No. 3,648,348 or by laminating a preformed film to preformed cubecorner elements. By way of example, such sheeting can be made using anickel mold formed by electrolytic deposition of nickel onto a mastermold. The electroformed mold can be used as a stamper to emboss thepattern of the mold onto a polycarbonate film approximately 500 μm thickhaving an index of refraction of about 1.59. The mold can be used in apress with the pressing performed at a temperature of approximately 175°to 200° C.

Useful materials for making such reflective sheeting are preferablymaterials that are dimensionally stable, durable, weatherable andreadily formable into the desired configuration. Examples of suitablematerials include acrylics, which generally have an index of refractionof about 1.5, such as Plexiglas resin from Rohm and Haas; thermosetacrylates and epoxy acrylates, preferably radiation cured,polycarbonates, which have an index of refraction of about 1.6;polyethylene-based ionomers (marketed under the name ‘SURLYN’);polyesters; and cellulose acetate butyrates. Generally any opticallytransmissive material that is formable, typically under heat andpressure, can be used. Other suitable materials for formingretroreflective sheeting are disclosed in U.S. Pat. No. 5,450,235 toSmith et al. The sheeting can also include colorants, dyes, UVabsorbers, or other additives as needed.

It is desirable in some circumstances to provide retroreflectivesheeting with a backing layer. A backing layer is particularly usefulfor retroreflective sheeting that reflects light according to theprinciples of total internal reflection. A suitable backing layer can bemade of any transparent or opaque material, including colored materials,that can be effectively engaged with the disclosed retroreflectivesheeting. Suitable backing materials include aluminum sheeting,galvanized steel, polymeric materials such as polymethyl methacrylates,polyesters, polyamids, polyvinyl fluorides, polycarbonates, polyvinylchlorides, polyurethanes, and a wide variety of laminates made fromthese and other materials.

The backing layer or sheet can be sealed in a grid pattern or any otherconfiguration suitable to the reflecting elements. Sealing can beaffected by use of a number of methods including ultrasonic welding,adhesives, or by heat sealing at discrete locations on the arrays ofreflecting elements (see, e.g. U.S. Pat. No. 3,924,928). Sealing isdesirable to inhibit the entry of contaminants such as soil and/ormoisture and to preserve air spaces adjacent the reflecting surfaces ofthe cube corner elements.

If added strength or toughness is required in the composite, backingsheets of polycarbonate, polybutryate or fiber-reinforced plastic can beused. Depending upon the degree of flexibility of the resultingretroreflective material, the material may be rolled or cut into stripsor other suitable designs. The retroreflective material can also bebacked with an adhesive and a release sheet to render it useful forapplication to any substrate without the added step of applying anadhesive or using other fastening means.

The cube corner elements disclosed herein can be individually tailoredso as to distribute light retroreflected by the articles into a desiredpattern or divergence profile, as taught by U.S. Pat. No. 4,775,219.Typically the groove half-angle error introduced will be less than ×20arc minutes and often less than ±5 arc minutes.

All patents and patent applications referred to, including thosedisclosed in the background of the invention, are hereby incorporated byreference. The present invention has now been described with referenceto several embodiments thereof. It will be apparent to those skilled inthe art that many changes can be made in the embodiments describedwithout departing from the scope of the invention. Thus, the scope ofthe present invention should not be limited to the preferred structuresand methods described herein, but rather by the broad scope of theclaims which follow.

What is claimed is:
 1. A lamina suitable for use in a mold for use informing retroreflective cube corner articles, the lamina having opposingfirst and second major surfaces defining therebetween a first referenceplane, the lamina further including a working surface connecting thefirst and second major surfaces, the lamina comprising: at least onecube corner element formed in the working surface, the at least one cubecorner element having a four-sided perimeter in plan view comprising afirst pair of opposed parallel sides and a second pair of opposedparallel sides, the second pair of opposed parallel sides beingobliquely disposed relative to the first pair of opposed parallel sides.2. The lamina of claim 1, wherein the first pair of opposed parallelsides is parallel to the first reference plane.
 3. The lamina of claim1, wherein the at least one cube corner element comprises a row of cubecorner elements, each such element having a four-sided perimeter in planview comprising a first pair of opposed parallel sides and a second pairof opposed parallel sides, the second pair of opposed parallel sidesbeing obliquely disposed relative to the first pair of opposed parallelsides.
 4. The lamina of claim 1, wherein the lamina measures betweenabout 0.025 and 5 millimeters in thickness.
 5. The lamina of claim 4,wherein the lamina measures between about 0.1 and 1 millimeter inthickness.
 6. A mold comprising the lamina of claim
 1. 7. A cube cornerarticle formed by at least one replication of the mold of claim
 6. 8.The article of claim 7, wherein the article is a retroreflective cubecorner sheeting.